In the Gulf of Mexico Region, a grand industry has been generated for the production and refining of hydrocarbons. Related to this industry are the production of oily wastes from exploration and production, typically containing residual diesel, which is used in the formulation of drilling muds.
On the other hand, this region has suffered many spills due to corroded piping. There are some oil fields older then fifty years old. Many of these are in marsh areas or in mangroves o other swamps. Since pipelines were installed to connect individual wells to separation batteries, and from there to refineries and petrochemical plants, the anaerobic corrosion was not considered to be important, and as a result, many pipelines are corroded and spilling oil. Lately, Petroleos Mexicanos has been installing pipeline with protection, or made of non-corrodible materials to take care of this problem. The kinds of soils more commonly affected are from low lying areas, this kind of soil has high concentrations of organic material and clay. Some contaminated sites are found in the recent coastal plain, in sandy soil. Although the number of these last sites is less, it is of greater concern due to the probable impact on aquifers, due to their high permeability.
In addition to the chronic spills caused by corrosion in the production fields, Petroleos Mexicanos in the refining area manages approximately 13 thousand kilometers of pipelines in the country which frequently are affected by spills due to clandestine robberies of oil from the pipelines, especially between Minatitlan, Veracruz and Puebla State.
There also exist dumps of semisolid oily waste in the region. Due to the climatic condition in the Gulf of Mexico Region (above all high precipitation), industrial landfills are not permitted, as in other parts of the republic, such as Mina N. L. (see Bremer, 1995 and the Official Mexican Norm NOM-CRP004-ECOL/1993). Due to this limitation, as well as the generation of large quantities of oily waste resulting from drilling muds, marshy vegetation and soils contaminated by spills, muds from systems for the treatment of waste water from petrochemical plants and refineries, the petroleum districts previously used oil well confinement pit. Normally, the used the main waste pit in an oil well that never produced petroleum, or an old well that does not produce and is capped. These pits were never designed to receive the quantity of waste put in them and were not constructed of impermeable materials. Many times all of the space in the pit was used up but waste was still deposited at the site, especially in the drilling platform, beside the pit. This inadequate management of wastes frequently resulted in run off and infiltrations of hydrocarbons in the nearby environment.
Besides the above mentioned sources, there exist effluents from petrochemical plants and refineries. Many of these have old API (American Petroleum Institute) type water treatment systems which separate less dense and denser fractions by gravity. Generally, they were not adequate to control the quantity of oil in the flow, resulting in the contamination of nearby areas, commonly marshes, canals, rivers and lagoons. These waters regularly contain salts from the petroleum deposit, in addition to hydrocarbons, which can adversely affect the marshes and water bodies.
In this region it is difficult to determine exactly the extension of land which is contaminated, due to political and economic interests in the region, exaggeration and the confusion which is to be found. In recent years the theme of “contamination” has been politicized and there exists an “industry of claims”. Just in Tabasco State, Beltran (1993) reports that there are approximately 7,200 hectares affected, of these more than 90% are in marshes or floodable areas. More recently, Petroleos Mexicanos, with help from the Mexican Petroleum Institute has identified approximately 300 hectares contaminated in the state and 178 pits which are contaminated with oily wastes are projected to be restored (LaJous, 1997). Much of this difference is due to the definition of “contaminated areas”, and the criteria used to determine it.
The levels of contamination vary much according to the sources of hydrocarbons and the age of the petroleum installations. In the extreme western part of Tabasco State, where the majority of installations are older, some around 50 years old, it is common to have hydrocarbon spills coming from corroded pipelines. In general, the extension of a spill is a stain of approximately 5 hectares, in which the concentration of hydrocarbons can reach up to 30% (Rodriguez, 1997).
En the more recent areas of petroleum activity, for example in the north and northeast of Tabasco State, the techniques used to protect the environment were bettered over the years. In this zone, the pipelines are newer, and many have a covering to reduce the corrosion. Due to this, the spills caused by corrosion of pipelines are much less. Additionally, the design, construction, and management of waste pits were improved and much fewer problems of infiltrations or runoff are to be found (Vinalay, 1998). In this part of the state it is rare to encounter zones with obvious stains of oil. Near the petroleum wells, the extension of hydrocarbons is normally less than one hectare and with concentrations less than 1000 ppm TPH (Total Petroleum Hydrocarbons, Dominguez, 1998).
It is worth mentioning that the new policy of Petroleos Mexicans is not to leave oily wastes in pits during the drilling of wells. When the phase of drilling in which inverse muds are require takes place, these are recycled in the most possible way; the spent muds and contaminated cuttings are deposited in a concrete pit with a laminate roof or in containers. Periodically, these wastes are recollected for treatment in a central site among various wells. However, there are many areas which have been contaminated by historical practices of the last century.
Besides the above mentioned activities related to exploration, production and refining of petroleum, there also exist various sites in the Mexican republic which are contaminated due to inadequate use of the products derived from petroleum. Among these include spills of fuels and lubricants in the great industrial areas dedicated to the manufacture of goods, such as can be found in Mexico City, Federal District, in Mexico State, Monterrey, N. L., Saltillo, Coahuila, Guadalajara, and various areas of the Bajio region, among others.
Furthermore, spills in thermoelectric plants in the country, bus stations and repair shops, airports and marine ports should be considered, due to the use of large volumes of lubricants and fuels.
It is important to look for appropriate technological solutions to the characteristic conditions of the country, in climatic, social and economical terms. In this context, it should be mentioned the value of developing remediation technologies as alternatives for the recovery of impacted ecosystems and agroecosystems, and to avoid problems of health risk. However, today almost all of the technologies used for remediation of contaminated soil or treatment of drilling cuttings are from abroad, which requires the importation of know-how, materials, reagents and sometimes machinery and personnel. This situation has consequences such as in the increase in costs, the creation of dependency from abroad, and many times these technologies are not implemented adequately. Various times it has been necessary to send remediation projects out to bid repeatedly due to the incompletion of contractors or due to the lack of service providers which are incompetent to realize the solicited task adequately, causing delays and increasing the administrative costs related to the remediation. On the other hand, it is common the bad management of imported technologies results in infertility in treated soils. For these reasons the effectiveness is frequently reduced, and the remediation contractors have great difficulty in meeting the established goals of their client (Petroleos Mexicanos) and the environmental authorities.
Below a description of the remediation technologies most used in the region is presented:
Confinement
This consists of the excavation of contaminated materials and the placement of these materials in a site prepared for their final deposition, said site having a series of engineering controls such as liners, leachate ponds, monitoring wells, etc., which assure that the contaminated material will never be in contact with biological receptors. It is important to select an adequate site for the location of this kind of remediation, one having low precipitation, with geological horizons that reduce possible migration, a depth to aquifer of hundreds of meters, without seismic activity and out of the paths of hurricanes and tropical storms. These conditions exist in few areas of the country, being one of the principal limitations of this technology. Currently, their is only one site (in Mina, Nuevo Leon) of this type at the national level, which means that contaminated materials must be transported large distances, practically doubling the cost.
Reuse—Recycling
In these kinds of technologies the contaminated material is neutralized and reutilized for another purpose, typically for construction. Drilling cuttings have been incorporated into raw materials on the fabrication of cement and bricks; in the process the hydrocarbons are incinerated. The principal limitation of this technology is the local market for these materials, which reduce the quantity of materials that a cement or brick plant can receive and treat. Another application of this technology has been the production of structural base for rural roads or parking areas, in Poza Rica and the Lazaro Cardenas Refinery (Minatitlan, Veracruz), using soil of other materials which are extremely contaminated with oil as raw material. This technology is appropriate only for very contaminated materials, which contain approximately 30% or more of hydrocarbons, and where the hydrocarbons are of a very viscous type. It is not applicable to the majority of spills or drilling cuttings.
Incineration
In the incineration of materials to be treated, they go by a conveyor to an oven, commonly rotary, or “rotary kiln”, in which they are heated to temperatures greater than 800° C. to completely burn all of the organic contaminants. The residue is the mineral fraction of the soil and some ashes; all of the organic substances are burned. This is effective to treat soil with any kind of organic contaminant including viscous crude oil, PBCS, dioxins, chlorophenols, and many pesticides. However, the cost is high. Abroad it is approximately $100-$500 USD per ton of treated soil. The resulting material is not fertile. For use in crops, pasture, or nature refuge, it is necessary to add mulches and to realize a recovery of the soil structure which increases the technology cost even more. There has been only one remediation project with this technology in the southern part of the country (Mexico), in the Cinco Presidentes Oil Field (Tabasco). During the project there were problems with the high humidity content and organic matter in the soil to be treated, and they had to re-run several batches of material through the oven more than once, increasing costs. Furthermore, there were problems in maintaining the equipment functioning due to local labor customs, and the lack of parts and trained personnel. Due to this experience this technology was discarded as a remediation technology by PEMEX Exploration and Production, South Region.
Thermal Description
In this technology the materials to be treated go by a conveyor to an oven where they are heated, but only to volatilization temperature, not incineration. The vapors are collected in an extraction hood and are incinerated in a smaller oven at a high temperature, or they are treated by other processes such as condensation, or reuse, among others. The mass to be incinerated is reduced, this being the vapors instead of all of the material, reducing energy costs. The process is less costly than incineration and completes with other technologies in terms of costs for small volumes. It is effective for volatile and semi-volitile organic contaminants: drilling cuttings, soil contaminated with gasoline, diesel, light and medium crude petroleum. The treated material still conserves some fertility; the majority of the soil organic material is not burned. None-the-less, this technology is complicated in the Gulf of Mexico region due to the high humidity in the soil, which implies a higher energy cost to be heated. On the other hand, the equipment is imported, and for this reason represents problems with logistics to obtain parts and trained personnel for its implementation, all of which increase operation costs. Likewise, there are not mobile equipment in the region, making it necessary to transport contaminated materials to a central treatment centre and thereby increasing costs.
Chemical Oxidation
This technology depends on the partial oxidation of the hydrocarbons and mineral fraction using non-specific oxidants. In some methods part of the hydrocarbons are mineralized. In others the oxidized minerals and hydrocarbons are united to stabilize the material. The reagent dosification, pH, homogenization, and curing of the mix are managed. Typically, this results in a reduction in the hydrocarbon concentration of only 60-70%, being inadequate for the treatment of materials with high concentrations. Commonly, the reagents added are toxic and cause problems in the soil, including infertility. The majority of chemical oxidation technologies depend on the importation of reagents making the remediation more costly.
Chemical Stabilization
In the chemical stabilization, chemical reagents are added to the contaminated material which result in pozzolanic reactions in said material, sometimes adding other kinds of reagents which work to improve the fixation, such as polymers, ashes, silica, hydrophobilizing agents, etc. The use of chemical substances, especially calcium oxides which promote pozzolanic reactions, is also used in the construction industry to firm up bland materials, permitting the construction of roads, buildings, etc. on such materials. Internationally, this technology has been used for the stabilization of contaminated materials, but in general in low concentrations. In Mexico, this is not currently being used for remediation of waste treatment.
Microencapsulation
In the microencapsulation the different phases of free oil, water and sediment are separated with gravity separators, similar to API separators. The recovered sediment is mixed in liquid form with a cationic surfactant, forming micelles of surfactant and the organic contaminant. A solution of silicates (or carbonates) is applied that precipitate on the cationic nuclei of the micelles forming small grains of silicates with the consistency of sand. This does not work well for organic soils but it does work for mineral soils with less than 10% of total organics. The material can have problems at temperatures above 40° C. and this aspect needs to be watched. The homogenization and dosification are very important. It is effective for all kinds of organic contaminants and even materials slightly contaminated with metals. The reagents used are mostly imported. In the southern part of the country there has been difficulty implementing this technology due to the fact that it requires close attention to detail for its application. Generally, the regional population is not detail oriented and there have been problems in materials mixing, frequently having to treat a batch of material more than once to decontaminate it, reducing efficiency in the process, and increasing costs. This technology has been abandoned by PEMEX Exploration and Production, South Region due to the problems, but it still is being used in a limited way by the Marine Zone for the treatment of drilling cuttings.
Bioremediation
Bioremediation uses native and introduced microorganisms that consume organic contaminants, reducing their concentration. Conditions such as humidity, nutrition, aeration, and mixing are managed to optimize microbial activity. This technology is relatively economical but it is slow, requiring a constant mixing of the material in treatment to maintain aerobic conditions. Furthermore, it requires large area for treatment. None-the-less, due to the relatively low cost in comparison with other physical-chemical remediation technologies, it is one of the most used technologies in the southern part of the country.
Biochemical Degradation
Biochemical degradation treatment consists of the application and mixture of different reagents in waste pits to promote a series of chemical and biological reactions. Commonly, at the outset, surfactants are added to the drilling cuttings to mobilize hydrocarbons and make them more available for subsequent treatment. If a significant quantity of free phase oil is produced, this is collected on the surface. Afterward, an oxidant such as hydrogen peroxide is applied to partially oxidize the hydrocarbons. Lastly, inorganic nutrients and “organic catalyzers”, such as manure are mixed into the material to promote biodegradation. All of these operations are carried out using excavators, or dredges, mixing the material sequentially in the same waste pit in which the material (such as drilling cuttings or other wastes) was originally stored. This method has an advantage over other methods because it is not necessary to construct a treatment cell, thereby reducing costs, area, and simplifying the logistics of remediation projects. Nevertheless, there are potential problems with this method due to the semisolid conditions which are created in the waste pit, and because the equipment used is not adequate to provide sufficiently aerobic conditions for the oleophilic microorganisms employed in the biological treatment phase. Because of this, the biodegradation rates are much reduced, prolonging treatment times, and increasing costs due to equipment rental, fuel, and labor. On the other hand, the waste pits in which these materials are encountered, are not impermeable, and the constant mix of materials in them, as well as the application of surfactants con provoke the mobilization and infiltration of hydrocarbons, contaminating aquifers, many of which are only a few meters below the soil surface in the southern region (from <1 m up to 5 m, in general). The other inconvenience of the technology, as with bioremediation, is the necessity to constantly mix the material during the biological treatment phase, which increases costs (machinery, fuel, labor).
Composting
It is a non-specific oxidation of vegetable tissues and organic contaminants carried out by fungi and actinomycetes. Concurrently, a spontaneous chemical polymerization of the subproducts takes place resulting in the formation of humus, a more stable form with low toxicity (humification). To implement this kind of treatment, organic conditioners are added and chemical and physical conditions (humidity, temperature, aeration, nutrition) are controlled. The application of this technology is limited due to the requirement of large quantities of organic amendments in the mixture (typically 70%) and logistic problems in obtaining the large volumes necessary. Furthermore, by adding such large volumes of material, the project costs are increased due to the need to also increase the amounts of reagents, machinery rental, fuel and labor. It has only been used in tow sites in the southern part of the county, in the Sanchez Magallanes Oil Filed, Tabasco, and in the Santa Alejandrina marsh, behind the Lazaro Cardenas Refinery in Minatitlan, Veracruz.
Phytoremediation
Phytoremediation is the use of plants to immobilize contaminants in the soil and subsequently detoxify the material by mineralization and humification processes in which plant roots as well as rhizosphere associated microbes participate. It is very slow and today it is not used in Mexico, however, there are preliminary experiments in tropical environments using above all grasses for the cleanup of soils contaminated with hydrocarbons (Zavala et al. 2002, Hernandez and Pager 2003). It does not work for contamination below the root zone, for example in a waste pit, and for certain kinds of contaminants there may be concern of contaminant migration to adjacent lands before the remediation of the site is accomplished.
Discussion on Process Function
Various physical, chemical and biological processes are involved in the functioning of this technology. During the initial treatment with calcium oxides, it is probable that a pozzolanic reaction takes place between these and clay silicates in the soil or drilling cuttings with the subsequent formation of calcium silicates, calcium aluminates and hydrated calcium aluminum silicates (LaGrega et al. 1996). This process has been used to stabilize clayey soils for use as construction material (McKennon et al. 1994), as well as for macroencapsulation of soils contaminated with hydrocarbons or other oily compounds, sometimes in combination with other additives (such as fly ash, silica, cement, polymers or hydrophobilizing agents, Shimoda et al. 1989; Masuda et al. 2001; Kao et al. 2000; Ritter 1995; Weitzman and Hamel 1989; DuPont 1986; Al-Tabbaa and Evans 2003).
Among patents of special interest are: McKennon et al. (1994), for solidification of clayey soil for construction purposes; Shimoda et al. (1989), which employs quicklime and/or dolomite in combination with a polymer (PFTE) for the stabilization of hydrocarbon contaminated soil, Masuda et al. (2001), which uses calcium oxides and derivatives thereof for the stabilization of soil contaminated with dioxins and PCBs, Kao et al. (2000), which combines a mixture of additives such as calcium oxides and a heating process in ovens which immobilizes organic and inorganic contaminants, and Ritter (1995), which adds hydrophobilizing agents with lime to render waste substances inoffensive. Nonethe-less, these methods only stabilize soil with relatively low concentrations of organic contaminants and result in only partial treatment. In none of these patents is the use of calcium oxides in combination with organic conditioners contemplated for the stabilization of materials contaminated with hydrocarbons or oily contaminants.
During the next phase of the treatment, when organic amendments are added, absorption is probably the principal process involved, at least initially. It has been shown that absorption of organic contaminants is soil is primarily due to the soil organic content (Chiou et al. 1979), especially the humic substances therein (Bogan et al. 2003), and several organic materials, mostly of vegetable origin have been used as sorbents for the treatment of waters and waste waters (Perez et al. 2002; Choi 1996; Varghese and Cleveland 1998; Deschamps et al. 2002). Among the patents of interest in this context, there is Perez et al. (2002), in which the use of sugar cane bagasse is mentioned for the treatment of petroleum contaminated water. Studies have also suggested that the type of humic materials in soil affect the sorption of these compounds (see Gauthier, 1987, for example) and have proposed the addition of refined humic substrates for remediation of xenobiotic contaminated soils (Fukushima et al. 2002).
Following the initial sorption of the hydrocarbons to the organic materials added, further biological and chemical processes are probably involved in the remediation. Some of the more available hydrocarbons are likely mineralized by oleophilic microorganisms, these kinds of hydrocarbons being readily biodegraded (Atlas 1986; Xie and Barcelona 2003). However, it is probable that a considerable portion of the hydrocarbons are sequestered by humification reactions thereby reducing the availability in the soil and the relative toxicity (Alexander 1995; Kelsey and Alexander 1997; Barr and Aust 1994; Paul and Clark 1989; Bohn et al. 2001; Ehlers and Luthy 2003; Bogan and Sullivan 2003; White et al. 1999).
Various investigators and inventors have also proposed adding organic materials to contaminated soil to stimulate microbial mediated treatment, by sorption into the organic amendment and subsequent decomposition of the vegetable organic matter and organic contaminant, either by mineralization or humification reactions (Breitenbeck and Grace 1997; Valo et al. 1991; Bradley et al. 1996; Grey et al. 2000; Gill 1996; Torstensson and Castillo 1997).
Important patents in this context are: Bradley et al. (1996), which suggest the use of sugar beet pulp enriched to increase its nitrogen content, in combination with white rot fungi for the decomposition of aromatic compounds, including polycyclic aromatic hydrocarbons (PAHs) and chlorinated aromatic compounds; Valo et al. (1991), on the treatment of soil contaminated with chorophenol, using Rhodococcus and Mycobacterium, in a process in which heat is generated by composting (to increase the biodegradation rate), and in which the bacteria are immobilized on a polyurethane support; Grey et al. (2000), which uses composting for the decomposition of the xenobiotic methoxychlor; and Gill (1996), which is about inoculating cotton waste with contaminated soil, composting the mixture, mixing the composted material with contaminated soil (a ratios of 1:1 to 1:5) plus adding chemical accelerators, and maturing the mixture at ambient temperature, as well as composting of organic material used as an absorbent for contaminants. In contrast to the chemical-biological stabilization in this proposal, these researchers have used primarily composting techniques, frequently using much greater amounts of organic amendment (on the order of 30-70% organic matter, instead of 1-10% as in this patent application), in a process that is qualitatively different; or they propose the application of modified organic amendments, instead of unmodified amendments.
During the maturation phase of the process, the role of the plant rhizosphere may also be important. Plant roots assist bioremediation in the soil by a number of processes. Initially, they are important because they assimilate organic substances, such as hydrocarbons, effectively immobilizing them among the roots. Subsequently, they stimulate microbial by exuding organic compounds that act as cosubtrates for oleophilic microorganisms. Also, when the roots grow they open up the soil, increasing aeration and improving drainage. Many of these processes also stimulate microorganisms that act in the humification of organic contaminants. For example, many rooty materials, especially lignin containing substances, can serve as food for actinomycetes and fungi, the principal microbes involved in the production of soil humus. Additionally, some plants have enzymes which initiate the breakdown of some hydrocarbons, especially the aromatic fraction (Zavala et al. 2002; Adams and Castillo 2004; Adams and Castillo 2000; Schnoor et al. 1995; Carman 1997; Carman et al. 1997; Drake 1997, Drake 1997a, Hernandez and Pager 2003). After a few months, it is common to observe a vigorous herbaceous growth, being an important aspect of this method, which demonstrates not only the detoxification of soil and drilling cuttings, but also improvements in the soil productivity as well.